Undesired butterworth lowpass filtering graph in C++

Hi all,

I am trying to remove a high frequency noise in ECG data using butterworth lowpass filter at normalized cut-off frequency of 0.05 Hz and sampling rate of 500. Using Matlab I can able to remove the noise successfully by following the above mentioned specifications. When I use same coefficients in my c++ code, I am not getting the desired output instead I am getting a random graph. I have been working it for the long to sort out but still I am clueless. Can anyone help me in sorting it out. Thanks in advance.

Answer 1

[Sivapriya Srinivasan]

I understand that you are able to remove high frequency noise in ECG data using MATLAB, but unable to implement the same using C++.

You can implement the Butterworth lowpass filter using digital signal processing library. Here is a sample code without ECG data array input:

#include <iostream> 

#include <vector> 

#include <dsp.h> 

#include <cmath> 

using namespace std; 

int main(){ 

 // number of samples in ECG data 

    const int N = 1000; 

// normalized cutoff frequency 

    const double f0 = 0.05;  

// sampling rate (Hz) 

    const double fs = 500.0;  

// cutoff frequency (Hz) 

    const double fc = f0 * fs / 2.0;  

// Your ECG data array input 

    double ecg_data[N] = { ... };  

    // initialize Butterworth lowpass filter 

 // filter coefficients 

    vector<double> b, a; 

// order of filter 

    const int order = 4;  

// calculate filter coefficients 

    butter(order, fc, fs, LOWPASS, b, a);  

    // filter ECG data 

    double ecg_filtered[N]; 

    for (int i = 0; i < N; i++) { 

        ecg_filtered[i] = 0; 

        // apply filter 

        for (int j = 0; j <= order; j++) { 

            if (i - j >= 0) { 

                ecg_filtered[i] += b[j] * ecg_data[i - j]; 

            } 

            if (i - j - 1 >= 0) { 

                ecg_filtered[i] -= a[j + 1] * ecg_filtered[i - j - 1]; 

}}} 

// output filtered ECG data 

    for (int i = 0; i < N; i++) { 

        cout << ecg_filtered[i] << " "; 

} 

    cout << endl; 

    return 0; 

}